Process of normalizing hot pressed plywood



Patented Mar. 14,1939

UNITED STATES 2.150.841 v rhoonss or NORMALIZING no'r mssnn I ruwoonlamest. Nevin, Aberdeen, Wash.

No mm. Application April z, 1988, serial No. 199,585

PATENT omen 4 assum APR 14.1942;

4 Claims. (01. 34-124) method of treating a press. This heat andpressure treatment converts' the thermo-setting glue into an insolubleand infusible bonding agent. permanently welding the veneer sheets tothe cores or cross banding and forming'a plywood panel. This hotpressing operation expands the wooden veneer sheets and cores and sincethe operation is usually carried out at temperatures above 212 F.,themoisture content of the wood is also greatly reduced.

The expansion of the wood veneers and-cores is greatest across the grainof the wood so that when the plywood'panelsare cooled afterthe gluesets, stresses will occur along the glueline of the panel. Thesestresses will build up or accumulate until a check or crack is formedalong the glue line. Such checks or cracks, of course, greatly diminishthe shear strength of the panel. If the panels are then subjected toweather conditions or to other than substantially bone dry atmospheres,they will warp and blister, due to an uneven moisture absorption intothe wood.

I have found that hot pressed plywood panels can be normalized toprevent the formation of .checks and blisters and to forestall possiblefuture warping. In accordance with this invention the hot pressedplywood panels are heat-treated in a hot, humid atmosphere immediatelyafter the hot pressing operation and before the same have cooled. Thistreatment plasticizes and softens the wood along the glue line torelieve stresses. At the same time, the wood absorbs moisture from thehumid atmosphere so that it will not warp under any weather conditions.

Humidification or treatment of the hot pressed panels in a moistatmosphere is not sufllcient to normalize the panels. I have found thatthe humid atmosphere must'be heated. A temperature range of from 150 to210 F. is desirable.

The humidity of this atmosphere should be maintained as high as possiblewithout actually condensing water on the panels. A relative humidityaround 98% is desirable.

It is, then, an object of thisinventio'n' to norm'ali'ze hot pressedplywood panels. A further object of this invention is to treat preventthe same from check- 4 hot pressed plywood panels forrelieving stressesdeveloped during the hot pressing operation and for imparting a moisturecontent to the wooden veneers and cores that will forestall warping ofthe panels. I

Another object of thisinventlon is to heat treat hot pressed plywoodpanels in a humid atmosphere immediately after the hot pressingoperation. v I

I Other and further objects of the invention will become apparent tothose skilled in the artfrom the following detaileddescrlption of thepreferred processand from the appended claims.

Natural wood veneer sheets to be used in the I manufacture'of hotpressed plywood panels are prepared in the usual way by peeling woodenlogs on a lathe and then drying the resulting peeled sheets. The sheetsare dried to a moisture content between 2 and 4%. 7 The sheetsselected'for plywood cores or cross banding are coated on both broadfacesther'eof with an aqueous solution of a cresylic acid-formaldehydepartial condensation product. The co'n-.

densation product is prepared by reacting metacresylic acid(meta-cresol) and formaldehyde exothermically inthe presence of causticsoda until somewhat constant temperatures around. 208-F. are reached.The reactionis then'arrest- .ed by the incorporation of a weak aqueousalkaline solution of sodium hydroxide. To insure the preparation of ,acondensation product that is soluble in water in all proportions, theratio of sodium hydroxide to meta-cresol should notbe less than 16 to100 parts by weight. Equivalent amounts of other alkali metal hydroxidescan be used.

The aqueous resin solution is preferably applied to thecore sheet by aseriesof coating" operations. Each coating is dried toa desired moisturecontent and the final coating may be utilized to impart a desiredmoisture content at the glue line of the plywood panel duringthe hotpress ng I operation.

The core sheets are sandwiched between veneer sheets to form a" stack ofthe desired numberof plies-u The stack is then placed between. the

platens of'a hot'press. Thepress is heated to temperatures around-330 to340 for converting 'the meta-cresol-form'aldehyde resin into a 'hard,infusible and insoluble binder permanently uniting the veneers to thecore. The stacks or piles in the press are subjectedtopressure varyingwiththe nature of the wood. Pressures of "120 lbs. per square'inch areused for Redwood sheets are heated to temperatures somewhat lower thanthe temperatures of the hot platens. This causes a drying of the wood aswell as an expansion of the same. The, wood will expand crosswise of itsgrain to a much greater extent than the expansion along the grain. Sinceadjoining sheets are arranged with the grains thereof running at rightangles to each other, stresses will occur along the glue lines betweenthe sheets.

After the resin has set, due to the hot pressing operation, the panelsare removed from the presses and, in accordance with this invention, are

placed in kilns where they are heat-treated in humid atmospheres beforethey cool.

The panels are preferably stacked between racks and weighted to keepthem in a flat condition while in the kilns.

' The kiln is heated to temperatures between 150 to 210 F. and theatmosphere therein is maintained at a high relative humidity.- Arelative humidity of from 90 to 98% is desirable.

The panelsremain in the kiln until the moisture content thereof israised preferably to above 6%. A range of 6 to, 12% moisture content isdesirable for the normalized product. The amount of time that the panelsmust remain in the kiln depends upon the number of plies in the paneland the thickness of the panel. The following table will illustrate asuitable schedule for the normalizing operation on three, five andseven-ply panels:

- Weight of Number Weight Percent 32:5 panel cm of hours of panelmoisture in panel panel tering humidileaving content kiln lied kiln ofpanel Inch: 7

5 a 157- 50 19 I. (X) 8. 03

After treatment at elevated temperatures in the humid atmosphere untilthe desired moisture content has been reached in the panels, the sameare cooled gradually before removal from the kiln. The panel should becooled to temperatures ,izing not above 150 1". beforethey are removedfrom the kiln.

From the above description it'should be underof hot plywood panels atelevat d temperatures in humid atmospheres to'impart a. moisturecontentbetween 6 to 12% to the panels. The humid atmosphere treatment must becarried out at elevated temperatures to plasticize that thisinventionrelates to the norma and soften the wood so that stresses setup during the hot pressing operation are relieved.

I am aware that numerous details of the process may be varied through awide range without departing from the principles of this invention,a'ind I, therefore, do not purpose limiting the patent grantedhereonotherwise the prior art.

than necessitated by I claim as'my invention:

1. The method of normalizing hot pressed ply-. wood panels composed of aplurality/0f bondedtogether wood plies with adjoining plies having thegrains thereof running at anangle to each other which comprises placingthe panels while still in a heated condition from the hot pressingplywood operation into a humid, heated atmosphere until the panelsabsorb an appreciable amount of moisture and then slowly cooling thepanels before removing the same from said heated atmosphere.

2. The method'of normalizing hot pressed plywood panels composed of aplurality of bondedtogether wood plies with adjoining plies havin thegrains thereof running at an angle to each other which comprisestreating the panels while still in a heated and expanded condition fromthe hot pressing operation in an atmosphere heated between 150 to210 F.and havinga high rela'-- tive humidity until said panels have a moisturecontent of from 6 to- 12%, and thereafter gradually cooling the panelsbefore removal from the heatedatmosphere. I

3. The method of normalizing hot pressed plywood panels composed ofaplurality of bonded-' together wood plies with adjoining plies havingthe grains thereof running at an angle to each other to relieve stressesset nup in the panels during the hot pressing operation and to humidifythe panels for preventing subsequent warping which comprises introducingthe hot pressed panels while the same are still in a heated conditioninto'a humid atmosphere maintained at temperatures between 150 and 210F. until the panels have a moisture content of at least 6%.

4. The process of normalizing hot pressed plypanels composed ofa'plurality of bondedtogether wood plies with adjoining plies having thegrains thereof running at' an angle to each other whichcomprises heattreating the panels while still in a heated and expanded condition fromthe hot 'pressing'operation at, temperatures between 150v to 210 F. in ahumid atmosphere having a relative humidity of from- 90 to 98% until 22dpanels have a moisture content not below s JAMES V. NEVIN.

